Taxonomy & naming
Melanotaenia klasioensis was described in 2015 in a paper authored by a team including Kadarusman, Renny K. Hadiaty, and Laurent Pouyaud, published in the journal Cybium (39(2): 99–130) under the title 'Eight new species of rainbowfishes (Melanotaeniidae) from the Bird's Head Region, West Papua, Indonesia.' It was one of eight new Melanotaenia named together in that work, part of a wave of descriptions that followed intensive survey of the freshwaters of the Vogelkop (Doberai) Peninsula. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia klasioensis Kadarusman, Hadiaty & Pouyaud, 2015; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. The Bird's Head region in particular harbours a dense cluster of narrow-range Melanotaenia, many separated by river basin or by individual stream. M. klasioensis is diagnosed against its Bird's Head congeners by a combination of meristic and morphometric characters rather than by any single striking trait: counts reported in the original description include a divided dorsal fin (V–VI spines in the first dorsal, then I,14–15 in the second), an anal fin of I,22–24, 36–37 lateral scales, and 18–20 gill rakers on the first arch, together with a relatively large interorbital and body width, a short pre-anal length, and long pectoral and pelvic fins. The dorsal-fin base protrudes vertically behind the anal-fin origin, falling between the first and third anal soft ray.
Morphology
Melanotaenia klasioensis is a small, laterally compressed rainbowfish. The maximum recorded length is about 3.5 in standard length, a figure based on the type specimen; FishBase notes this measure is provisional and likely to be refined as more material accumulates, so it should be read as an early estimate rather than a settled maximum. As in other members of the genus, the body is compressed from side to side and carries the family's characteristic divided dorsal fin — a short anterior portion followed by a longer posterior one.
The clearest colour feature noted for the species is a red margin stripe along the dorsal fins, which holds up in well-conditioned adults. Beyond that, M. klasioensis is described mostly through its proportions and counts rather than any dramatic body pattern of the kind seen in showier rainbowfishes. Hobbyists raising tank stock report that adult males develop a bright, iridescent stripe along the dorsal ridge and intensify their colour during display; females and juveniles stay plainer. That degree of sexual dimorphism is ordinary for the genus. Wild-colour data beyond the dorsal red margin are sparse.
Habitat
Melanotaenia klasioensis is, on current knowledge, endemic to Klasio Creek in the Bird's Head (Vogelkop) region of West Papua, Indonesia — the type locality and the only place from which it has been recorded. It is a freshwater, benthopelagic fish of tropical streams, occupying the running and still waters of a single creek system on the peninsula.
Known from just one stream, the species' habitat can only be described in general terms. Like other Bird's Head rainbowfishes it would be expected in clear, flowing or slow-moving freshwater with marginal cover and vegetation. Published measurements of temperature, pH, and other water parameters for Klasio Creek itself are not available in the consulted sources, so its environmental envelope in the wild stays data-sparse. The single-creek distribution is the central fact of its biology. There is essentially no documented population anywhere else, which both limits what is known of its range and makes that one site decisive for its survival.
Feeding
No dedicated diet study for Melanotaenia klasioensis appears in the consulted sources, and FishBase places it at a trophic level of roughly 2.9 — consistent with a small omnivore that takes a mix of animal and plant matter rather than a strict predator. Rainbowfishes this size are opportunistic feeders in the upper and middle water column, taking small aquatic and terrestrial invertebrates, insect larvae, and some algal and plant material. M. klasioensis is reasonably assumed to follow suit, though direct wild-feeding observations are not documented here.
In the aquarium the species is reported to be undemanding and not a picky feeder, accepting most standard prepared and frozen foods. Breeder notes recommend a base of soft foods — quality flake, frozen items, and gel-type preparations — with hard pellets offered only in small or softened form, as hard food is said to cause digestive problems in this fish. A varied diet supports both colour and condition, as it does across the genus.
Mating
Courtship observations specific to Melanotaenia klasioensis are limited, but the genus has a consistent reproductive style, and the behaviour reported from tank stock fits that pattern. Adult males are the colourful, display-driven sex: they develop a bright iridescent dorsal-ridge stripe and 'flash' their colour during interactions, intensifying when courting and when competing with rival males. Females and juveniles stay comparatively plain.
In rainbowfishes generally, a displaying male stakes out a small area, courts a receptive female with flared fins and heightened colour, and leads her into fine vegetation to spawn. Display peaks in the morning and is amplified by the presence of other males, so the fish is at its most colourful in a group rather than as an isolated pair. Spawning observations specific to M. klasioensis beyond this general framework are not documented in the consulted sources.
Breeding
Melanotaenia klasioensis is bred in captivity — it is offered in the trade as aquarium bred-and-raised stock rather than wild-collected fish — but no detailed published breeding protocol turns up in the consulted sources, so the account here stays general. Rainbowfishes of this genus are egg-scattering plant spawners with no parental care: conditioned adults deposit small adhesive eggs that hang by fine filaments from fine-leaved plants or synthetic spawning mops, releasing a few eggs at a time over many days rather than a single clutch.
FishBase rates the species' resilience as medium, with an estimated population doubling time of roughly 1.4 to 4.4 years — in conservation terms, moderate recovery potential under good conditions. An established captive line, sold as tank-raised, confirms the species reproduces reliably in aquaria. As with related rainbowfishes, breeders would harvest eggs or mops away from the adults, which eat eggs and fry, and start the small fry on very fine first foods before moving them to newly hatched brine shrimp. Incubation times and fry-rearing details specific to this species are not documented here.
In the aquarium
Breeders describe Melanotaenia klasioensis as peaceful, active, hardy, and easy to keep, with the colour and behaviour typical of a small-to-medium rainbowfish. It is a rapid, restless swimmer, and the advice from those raising it is to give it room: a tank at least about 4 ft long, on the order of a 200-litre (55-gallon) aquarium, suits an active group, with larger Melanotaenia reaching roughly 5–6 in. As with the genus generally, it shows best and stays most colourful in a group, where the males display against one another.
Water-parameter guidance from a breeder's own tanks — offered as the conditions their stock thrives in rather than as a strict requirement — centres on a temperature near 79 °F and distinctly alkaline, hard water (pH around 8.3, with carbonate and general hardness above roughly 300 ppm). The species reportedly tolerates a range of hardness provided things stay stable, so consistency matters more than hitting exact numbers. With no confirmed wild-habitat measurements for Klasio Creek, these aquarium figures are the most concrete husbandry reference at hand. FishBase rates the species harmless and flags its aquarium potential.
Conservation
Melanotaenia klasioensis is assessed as Critically Endangered (CR) on the IUCN Red List under criterion B1ab(iii), with the assessment dated 4 December 2019 (IUCN species ID 161098833). The listing follows directly from the single-creek distribution: a fish known only from Klasio Creek has an extremely small range and no outlying population to buffer it against local decline, so any degradation of that one habitat threatens the whole species. The CR criteria invoked reflect a small extent of occurrence combined with a continuing decline in habitat quality.
The IUCN assessment identifies the specific threats as the expansion of oil palm plantations, the burning of forest for small-scale agriculture, and the invasive snakehead (Channa striata), with the extent of occurrence estimated at just 25 mi² at a single location — the same kind of pressure behind the critically endangered status of several narrowly endemic Bird's Head Melanotaenia. Against that backdrop the captive population is a real safeguard: M. klasioensis is propagated and traded as tank-bred stock, which keeps it established outside its single wild creek and reduces collecting pressure. Aquarists support its conservation simply by sourcing captive-bred fish. The species is not evaluated under CITES or CMS.